Field of Invention
This invention generally relates to a quantum rod film used in backlight unit of liquid crystal display for enhancing the gamut and light utilization of the liquid crystal display.
Description of Related Art
Polarizers commonly used in the liquid crystal display are absorptive polarizers. In the liquid crystal display, when the non-polarized light emitted from the backlight is incident onto the absorptive polarizers, a component of the incident light parallel to the absorption axis direction of the polarizers is absorbed and cannot pass through the absorptive polarizers. Therefore, after the light emitted from backlight passing through the absorptive polarizer, the light will lose at least 50%. In addition, after the light further passing through the electrode layer, color filter, liquid crystal module and glass substrate, only less than 10% of the light originally emitted from backlight can be transmitted to the observer. Accordingly, the light utilization of the backlight is quite low.
Several approaches to enhance the light utilization of the backlight have been proposed. For example, a brightness enhancement film (e.g., Dual Brightness Enhancement Film, DBEF) and/or a prism film may be used in backlight unit for continuously refracting and reflecting to recirculate and recycle the light which is unable to be transmitted by the polarizer. The light can be redirected out of the backlight unit to enhance the brightness of the backlight unit. In another example, concentrating the light of large viewing angle area of the liquid crystal display can increase the luminance at viewing direction. However, those solutions may increase the luminance of the backlight unit of a liquid crystal display; no significant effect is provided to the gamut and the color saturation of the liquid crystal display.
Therefore, a solution is proposed by using quantum dots in backlight source, to increase the gamut thereof. The quantum dot is a semiconductor material of zero-dimentional structure, and it is able to absorb UV light or blue light having shorter wavelength and emit green light or red light having longer wavelength to mix to emit white light source. Because the excitation spectrum of the quantum dots material is with a narrower full-width-at-half-maximum (FWHM), the gamut area of the liquid crystal display using the quantum dots will be more than 100% NTSC.
In addition, it is also proposed another approach that a quantum rod layer is integrated into the backlight unit. The quantum rod is a nano-scale semiconductor material. It is in a shape of a one-dimensional rod-like structure. The quantum rod film is different from the absorptive polarizer which absorbs non-polarizing fight with evolution of heat. The quantum rod is able, to absorb the non-polarized light to emit a polarized light with a wavelength longer than the original non-polarized light from the major axis direction thereof. Because of the high internal quantum efficiency, most of the incident light from the backlight source is polarized. The quantum rods are aligned in a direction of major axis, and the emitted polarized light is efficiently passed through the transmission axis of the polarizer disposed on the liquid crystal display. Accordingly, compared to the backlight unit with quantum dots, the light utilization of a backlight unit with the quantum rods will further be enhanced.
Usually, the dichroic ratio (DR) is used to evaluate the efficiency of transformed polarizing light emitted by quantum rod film. The dichroic ratio is obtained by an equation DR=Y///Y⊥, wherein the Y// is the transmittance obtained as the major axis of the quantum rod film is parallel to the transmission axis of the detection polarizer, and Y⊥ is the transmittance obtained as the major axis of the quantum rod film is perpendicular to the transmission axis of the detection polarizer. When a light is not transmitted through a quantum rod film, the Y// and Y⊥ are almost the same and thus the dichroic ratio is about 1. As the dichroic ratio is higher, the dichroism of the quantum rod film is significant. When a light is transmitted through a quantum rod film with a higher dichroism, the light will be transformed into a light with a better polarization and directionality. As using a quantum rod film in the stacked optical films of the current backlight unit, light will be reflected and refracted in and out of the optical films or be scattered by the particles composed in films such that dichroic ratio of the light excited by the quantum rod film passing through these optical films is going to be decreased. Thus, when the light generated from the backlight module with a quantum rod film passes through the polarizers of the liquid crystal display, the brightness of the display is not as expected.
Furthermore, as shown in FIG. 1 in the conventional quantum rod film 1 utilizing blue light backlight source (LB) to emit red light (LR) and green light (LG), when the major axes of the quantum rods 2 are aligned in X-axis direction, the component of the blue light backlight source (LB) in X-axis is absorbed by quantum rods 2 to emit red light (LR) and green light (LG) in the same X-axis direction, the component of the blue light backlight source (LB) in Y-axis is most directly transmitted through the quantum rods 2 as transmitted blue light (LB1). Since the polarization direction of the transmitted blue light (LB1) is different from that of the emitted red light (LR) and green light (LG), part of the transmitted blue light (LB1) may not pass through the transmission axis of the polarizer of the liquid crystal display, the light utilization of the transmitted blue light (LB1) is lower. When the more transmitted blue light (LB1) passes through the transmission axis of the conventional quantum rod film 1, the Y⊥ component is more such that the dichroic ratio of the conventional quantum rod film 1 is lower. Furthermore, if most of the blue light backlight source (LB) directly passes through the quantum rods 2, the red light (LR) and green light (LG) are emitted less, and the light intensity will be insufficient. Therefore, it requires increasing the amount of the quantum rods 2 to maintain the desired color coordinate of mixed white light. The cost of the quantum rod film will increase. There is a demand for a novel quantum rod film which decreases the transmitted blue light (LB1) in Y-axis and increases the excitation number of the quantum rod 2 excited by the blue light backlight source (LB) and the blue light utilization.